Hitherto, liquid barium/cadmium stabilizers have normally been used in the thermoplastic processing of semirigid PVC and flexible PVC to form films by the calendering process. However, since toxic heavy metals, such as cadmium, are no longer acceptable today for reasons of factory hygiene and ecology, stabilizer systems based on liquid barium/zinc stabilizers have been increasingly used for the application mentioned. However, since the heavy metal, barium, also shows relatively high toxicity, there is a general trend in practice to replace these barium/zinc stabilizers also by physiologically safe calcium/zinc stabilizers.
In the calendering of flexible and semirigid PVC, the stabilizer system used has to meet stringent requirements. On the one hand, good early colors coupled with adequate color hold are required; on the other hand, the calendering process and the film properties often additionally required, such as good weathering resistance, not only have to guarantee excellent long-term stabilities, they are also expected to ensure problem-free regenerate processing. In addition, the stabilizer systems used should also reduce the tendency towards blocking and, hence, should enable the film to be removed more easily from the hot calender rolls and should have such rheological properties that the films are free from flow structures and do not have any defects. All these requirements have to be viewed in particular against the background of the high calendering speeds now typically encountered in practice. Another crucial requirement which stabilizers used for this purpose are expected to satisfy is the following: they should not cause any plate-out on calender rolls, take-off and cooling rolls or on any following embossing and laminating units.
The liquid Ba/Cd and Ba/Zn stabilizers normally used for the production of semirigid and flexible PVC films adequately satisfy the requirements discussed above. Although Ca/Zn stabilizers do not cause any plate-out on calender rolls, they often show only inadequate thermal stability and long-term stability. By contrast, hitherto known solid Ca/Zn stabilizers show the required long-term stability coupled with good early colors and color hold providing relatively large quantities of metal soaps are used. Unfortunately, stabilizers such as these cause such serious plate-out on calender rolls that they cannot be used in practice for the production of calendered films of flexible or semirigid PVC.
On the subject of known stabilizer systems containing calcium and/or zinc salts of fatty acids, reference is made for example to the following publications:
DE-A-31 13 442 relates to stabilized PVC molding compounds containing a stabilizer combination based on alkali metal, alkaline earth metal and/or zinc soaps, co-stabilizers, fine-particle crystalline water-containing alkali metal alumosilicates, lubricants and, optionally, other typical additives. Crucially, these stabilizer combinations--in addition to an alkali metal alumosilicate--must also contain a co-stabilizer selected from .beta.-diketones, .alpha.-ketoenol esters, .alpha.-acyl lactones, substituted 1,4-dihydropyridine dicarboxylic acids and optionally substituted pyrroles. In one preferred embodiment, calcium soaps are used in conjunction with zinc soaps.
EP-A-0 027 588 describes stabilizer combinations for PVC molding compounds which contain the following components: fine-particle crystalline water-containing sodium alumosilicates, calcium salts of fatty acids, zinc salts of fatty acids, partial esters of fatty acids and polyols and thioglycolic acid esters of polyols and/or monohydric alcohols.
Finally, DE-A-41 34 325 relates to a process for stabilizing polymers based on chlorine-containing olefins using a stabilizer mixture containing the following components: calcium complexes of 1,3-diketones, zinc and optionally calcium salts, .beta.-diketones, hydrotalcites, sodium alumosilicates and/or calcium/aluminium hydroxyphosphites and also polyols and/or hydroxyfunctional isocyanurates.
However, it has been found in practice that, where the stabilizer combinations disclosed in the prior art are used for the production of semirigid and flexible films of PVC by the calendering process, not all the requirements mentioned above are adequately satisfied. In particular, the formulations in question cause plate-out on calender rolls.
By contrast, the problem addressed by the present invention was to provide a stabilizer combination for the production of semirigid and flexible films of PVC which would meet the stringent thermal stability requirements of the calendering process (good initial color, color hold and long-term stability) and which, at the same time, would not cause any plate-out on the calender rolls or following machinery, such as take-off, laminating and embossing units.